fix(scheduler): drop cross-task mutex, quiet window + retry

Incident: evening full snapshot could be recorded as ok with
already_completed/elapsed=0 after mutual skip with hourly sync, then
next_run jumped to the next day.

- Full-site snapshot never skips because another task is running
- 23:00-01:00 Asia/Shanghai: defer discovery / snapshot_recent /
  candidate_cleanup to after 01:00; fans never defers
- Skip records ok=false + skipped=true (not success / not completed)
- Skip/fail reschedule soon (~5m); daily success still next snapshot_at
- TaskControl return value drives next_run; system UI shows skip fields

tests/test_scheduler.py

@@ -8,7 +8,7 @@ import threading
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 sys.stdout.reconfigure(encoding="utf-8")
 
-from reaction_board.scheduler import ScheduledTask, Scheduler  # noqa: E402
+from reaction_board.scheduler import ScheduledTask, Scheduler, TaskControl  # noqa: E402
 
 
 def test_run_when_due_and_reschedule():
@@ -128,9 +128,34 @@ def test_next_run_fn_controls_reschedule():
     assert calls == [1_000.0, 1_123.0]
 
 
+
+def test_task_control_overrides_ok_and_next_run():
+    clock = [1_000.0]
+    task = ScheduledTask(
+        "defer",
+        60,
+        lambda: TaskControl(ok=False, next_run=1_500.0, error="quiet"),
+    )
+    scheduler = Scheduler([task], clock=lambda: clock[0])
+    results = scheduler.run_due()
+    assert len(results) == 1
+    assert results[0].ok is False
+    assert results[0].error == "quiet"
+    assert task.next_run == 1_500.0
+    # 未到 next_run 不重跑
+    clock[0] = 1_499.0
+    assert scheduler.run_due() == []
+    clock[0] = 1_500.0
+    task.fn = lambda: TaskControl(ok=True, next_run=2_000.0)
+    results = scheduler.run_due()
+    assert results[0].ok is True
+    assert task.next_run == 2_000.0
+
+
 def main():
     test_run_when_due_and_reschedule()
     test_next_run_fn_controls_reschedule()
+    test_task_control_overrides_ok_and_next_run()
     test_task_error_does_not_block_other_tasks()
     test_run_forever_with_stop_event()
     test_max_iterations_and_validation()